ABSTRACT
Flood is one of the most dangerous natural
disaster in Vietnam. Assessing flood hazard is a
long term ambition of the society, especially in
low-land cities where almost its communities expose to flood caused by heavy rainfall over its
upstream river basin. In order to do that, designing flood events is one of the very first step.
This paper evaluates some methods of flood design and give an advise for choosing relavant
method in Vietnam which have been test in Vu
Gia Thu Bon river basin. The procedure includes
several steps: 1. Design a storm event which
cause heavy rainfall over the basin; 2. Estimate
the Arial Reduction Factor (ARF); 3. Estimate
the flood peak; and 4. Design the flood events.
The first step have been done by develop IDF
curve over the basin; then several combination
methods of Arial Reduction Fator and flood peak
estimation have been applied and evaluated to
choose the most relevant one with respect to literatural flood peak values. The result show that,
USWB method for ARF identification in combination with Rational method for flood peak estimation give a very good result for flood hazard
design.

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62
Vietnam Journal of Hydrometeorology, ISSN 2525-2208, 2019 (03): 62-70
Truong Van Anh1, Le Thu Trang1
ABSTRACT
Flood is one of the most dangerous natural
disaster in Vietnam. Assessing flood hazard is a
long term ambition of the society, especially in
low-land cities where almost its communities ex-
pose to flood caused by heavy rainfall over its
upstream river basin. In order to do that, de-
signing flood events is one of the very first step.
This paper evaluates some methods of flood de-
sign and give an advise for choosing relavant
method in Vietnam which have been test in Vu
Gia Thu Bon river basin. The procedure includes
several steps: 1. Design a storm event which
cause heavy rainfall over the basin; 2. Estimate
the Arial Reduction Factor (ARF); 3. Estimate
the flood peak; and 4. Design the flood events.
The first step have been done by develop IDF
curve over the basin; then several combination
methods of Arial Reduction Fator and flood peak
estimation have been applied and evaluated to
choose the most relevant one with respect to lit-
eratural flood peak values. The result show that,
USWB method for ARF identification in combi-
nation with Rational method for flood peak esti-
mation give a very good result for flood hazard
design.
Keywords: Flood design, Vu Gia Thu Bon,
Flood hazard, Flood risk.
1. Introduction
Flood is one of the most dangerous natural
disaster in Vietnam (Assistance, 2018). Assess-
ing flood hazard is a long term ambition of the
society, especially in low-land cities where al-
most its communities expose to flood caused by
heavy rainfall over its upstream basin. The very
first step of hazard assessment is designing flood
scenarios. In a literature, a design flood is a hy-
pothetical flood (peak discharge or/and hydro-
graph depending on the purpose of each study)
adopted as the basis in engineering design of a
water resources system (Jain, 2003). The two
most used-approaches for generating the design
flood are flood frequency analysis (FFA) and
rainfall - runoff analysis (RRA) (Daniel and
Wright, 2016). The first one designs a flood via
statistical analyses of the observed discharge
data. This method is usually used to estimate
peak discharge at a certain location during a
flood design event. The second one designs a
flood by estimating the runoff from design rain-
fall event which is induced by statistical analyses
of observed rainfall data. This method is usually
used to design the peak and hydrograph of an ex-
pected flood event.
For many developed countries like US or
Research Paper
IDENTIFICATION OF RELEVANT METHOD FOR FLOOD EVENTS
DESIGN AN APPROACH TO FLOOD HAZARD ASSESSMENT AT
RIVER BASIN SCALE
ARTICLE HISTORY
Received: November 08, 2019 Accepted: December 18, 2019
Publish on: December 25, 2019
TRUONG VAN ANH
Corresponding author: truongvananh.vn@gmail.com
1Hanoi University of Natural Resources and Environment
w
τ is the lifetime
o
L
H
DOI:10.36335/VNJHM.2019(3).62-70
63
western regions, they use FFA to estimate the de-
sign floods because they have densed discharge
stations which cover almost represenative loca-
tions in their river basins (Survey, 2006; Hy-
drology, 1999; Hydrology, 2012; Engineers,
2001). However, in the developing country like
Vietnam, where the observed data is usually not
long enough for frequencies analysis, the FFA
can cause a bias error. Infact, many authors
found that the RRA is more reliable than the FFA
when applied to the basin with fews observations
(MCKerchar and Macky, 2001; Calver et al.,
2009; Lee et al., 2011). That is why RRA is rec-
ommended to use in many regions in the world.
Vietnam have been issue some technical star-
dard on flood design for the purpose of engi-
neering design at the site without data such as
TCVN 9845:2013 on Calculation of flood flow
characteristic which usually used to design trans-
portation structures or TCVN 7957:2008 on
Drainage and sewerage - External Networks and
Facilities - Design Standard. The first one guides
to estimate the flood peak based on the rain
height of given frequencies and use a referent-
historical-flood for scaling flood peak and defin-
ing the hydrogaph. The second guides to design
IDF curve over the basin to estimate the rain
height of certain frequency needed to be drained
in urban area. Both cases give a difficult ap-
proach for analyzing the flood hazard at the large
basin scale where the rainfall is spacially dis-
trubuted. In Vietnam, the engineer usually
choose a reference storm event which happened
in the past and be scaled up to the relevant value
of design frequency such as 10, 20, 50 or 100
year return periods based on the purpose of the
studies. However as we all know, the storm is
stochastic event which can not be happen twice
in reality. In addition, in flood hazard analysis,
the extreme flood is the one contributed by rain-
fall over the whole basin. This paper introduce a
procedure for flood designing using RRA ap-
proach for supporting flood hazard assessment.
This procedure will be tested on Vu Gia Thu Bon
River basin.
2. Method
2.1 Description of study site
Vu Gia Thu Bon River basin is one of the four
biggest basins in Vietnam. Base in the Central
part of Vietnam and cover the part of Kon Tum,
Da Nang and Quang Nam provice, its delta usu-
ally face to flood due to its special topography
and geographic location (Fig. 1). It has the area
of about 10,350 km2. Only approximately 15%
its area is low land delta where collects all water
from its upper basin when they cover by a storm.
That is why the delta annually surffer to inunda-
tion and flooding which have been caused
human lost and extreme damage in Da Nang and
Quang Nam every year. Therefore the study of
flood hazard is valuable for this region. How-
ever, the mornitoring sites and observed data in
this basin are till scarce. There are only two dis-
charge stations in the basin: Nong Son in Thu
Bon river and Thanh My in Vu Gia river which
are located in the upstream of the system (Fig.
2). Therefore, FFA is difficult application in the
basin. This situation is being a case of almost
river basins in Vietnam where the data is scarce
and short. Hence, to analysis the flood hazard,
we should use DRRA method and start from
rainfall data instead of discharge data.
Truong Van Anh et al./Vietnam Journal of Hydrometeorology, 2019 (03): 62-70
Identification of relevant method for flood events design an approach to flood hazard assessment
at river basin scale
Fig. 1. Geographical location and topographic map of Vu Gia Thu Bon basin
2.2 Methodology
The methodology of flood design for flood
hazard assessment at river basin scale is the RRA
approach. Starting from rainfall analysis, the
hourly data for 20 - 30 years should be collected
and make the frequency analysis of the event
with different durations from 10 mins upto 72
hours based on the time concentration of the
sub-basin. The procedure is presented in Fig. 2.
Step 1: Design point rainfall
Current approach of analyzing the point rain-
fall at each station within and vincini the basin is
using Itensity Duration Frequency curve ( IDF )
of rainfall data at gauged station. Each curve
shows the intensity of rainfall during specific du-
ration at a given frequency. In this study, the
DDF curves were developed instead of IDF
curve for rainfall design purpose, referring to the
rain height instead of the rain intensity for easier
use in following phases, as described by Eq. 1.
where h is the rainfall depth (mm) for the du-
ration t; a, n are parameters to estimate from the
data series; then i = h/t is the rainfall intensity.
DDF curves are computed using this procedure
for 10, 20, 50, 100 and 200 years return period
for each available station in the basin area using
the set of parameters a &n specified for each rain
station.
Step 2: Design arial rainfall
After having point DDFs at each station,
transformation of point rainfall to areal rainfall
can be made by interpolating spatial the param-
eters of Depth-duration-frequency curves and
applying an empirically-derived areal reduction
factors (ARFs). Usually, the regionalized rain-
fall over the sub catchments can be estimated by
some popular methods such as Thiessen poly-
gon, gauged rainfall average, etc. In this study, to
overcome the lack of measured data and make
an homogeneous analysis for the whole basin,
maps of regionalized DDF curves parameters
(a&n)were developed, similarly to the method
proposed in the paper of (Nhat et al., 2008) for
ungauged areas.
For each sub-basin, rainfall critical height ac-
cording to various RP (100, 50, 20, 10) is evalu-
ated based on the DDF curves (h=axtn),
considering a duration t equal to concentration
time tc. An area reduction factor is applied to re-
sulting height, considering USWB formula
(from U.S. Weather Bureau with coefficients re-
6
(1)
64
65
calibrated by Benaglia (1997):
This formular will be valid as the best per-
forming concerning flood peak estimation.
Step 3: Design hyetographs
Design hydetographs are developed from de-
sign rainfall event which occur in the duration
equal to concentration time of the basin. Con-
centration time can be estimated by some em-
pirical formula, such as SCS formula or
Giandotti formula, etc. These methods require
some basin's characteristics defined from DEM
and land use maps to extract the area, mean ele-
vation, mean slope, hill slope sides of each sub
basin, etc.
Step 4: Design hydrographs
By applying a conceptual rainfall-runoff
model (rational model). According to this model,
the hydrograph shape is triangular, with a cen-
tral peak and a total time equal to double the con-
centration time of the sub-basin.
'/::)'/')*(/ '/=B(=/1 /
(2)
Fig. 2. Flood design procedure
Fig. 3. Solution for each step of the design flood procedure
3. Results and discussion
Vu Gia-Thu Bon River basin is devided into
30 sub-catchments (Fig. 4) which can be ana-
lyzing as one unit of hydrograph of a flood event
and concatinate each with the other to create the
flow of whole system.
Truong Van Anh et al./Vietnam Journal of Hydrometeorology, 2019 (03): 62-70
Fig. 4. Sub-basins defined in Vu Gia-Thu Bon River basin for flood analysis
Step 1: Design point rainfall: Designing
heavy rainfall events at rain stations
Within the Vu Gia-Thu Bon basin, only ob-
served data of discharge at the stations Nong Son
on the Thu Bon river and Thanh My on the Vu
Gia are available. Therefore, only two sub-basins
are considered for hydrological models' calibra-
tion and validation and design flood peaks. Other
sub-basins have to be estimated from rainfall.
This is the reason why IDF curves of rainfall at
all rainfall stations have been built to estimate
the discharge peaks of flood events.
For homogeneous analysis, the flood peaks at
Nong Son and Thanh My are also estimated
based on the rainfall events extracted from IDF
curve. A total of 15 rainfall stations within this
basin is available, as shown in Fig. 5.
In this study, the DDF curves were developed
for rainfall design purpose, referring to the rain
height instead of the rain intensity for easier use
in following phases, as described by Eq. 1.
DDF curves are computed using this proce-
dure for 10, 20, 50, 100 and 200 years return pe-
riod for each available station (Fig. 6) in the
basin area using the set of parameters &n spec-
ified for each rain stations.
Fig. 5. Rainfall stations in Vu Gia-Thu Bon River basin
Identification of relevant method for flood events design an approach to flood hazard assessment
at river basin scale
66
67
Truong Van Anh et al./Vietnam Journal of Hydrometeorology, 2019 (03): 62-70
23 2&3
Fig. 6. DDF curves for Tra My (a) and Da Nang (b)
Step 2: Design arial rainfall: Estimating rain-
fall spatialization over each subbasin
Usually, the regionalized rainfall over the sub
catchments can be estimated by some popular
methods such as Thiessen polygon, gauged rain-
fall average, etc. In this study, to overcome the
lack of measured data and make an homoge-
neous analysis for the whole basin, maps of re-
gionalized DDF curves parameters (a&n)were
developed, similarly to the method proposed in
the paper of Nhat et al. (2006) for ungauged
areas.
The validation was made with rain gauges ad-
ditional to those used for DDF curves estimation.
Fig. 7 presents an example of contour maps of a
and n parameters under 10-year return period.
Then the rainfall heights (Fig. 8) show a more
regular and gradually varied distribution on the
basin area, as the combination of a and n values
tend to attenuate the steeper gradient that can be
observed in some area from the contour maps. In
any case, the absolute variations in a, n parame-
ters and in obtained rainfall heights are not too
relevant between considered gauging stations in
the basin area, therefore the use of a regional-
ization procedure can provide good results.
Fig. 7. Spatial values of “a” (left side) and “n” (right side) of 10 year RP
Fig. 8. Distribution of maximum rainfall height for a 12 hour duration and 10 year return period
Step 3: Design hyetographs: ARF values as-
signed for each sub-basin
For each sub-basin, rainfall critical height ac-
cording to various RP (100, 50, 20, 10) is evalu-
ated based on the DDF curves (h=axtn),
considering a duration t equal to concentration
time tc. An area reduction factor is applied to re-
sulting height, considering USWB formula.
Infact, other formulas were tested in pilot
basin, as Wallingford formula and a formula
cited by Mekong River Commission Secretariat,
applied in Cambodia. The latter is the only ARF
formula that is found developed in South East
Asia, but it is meant for small basins, giving neg-
ative values for A > 2500 km2. USWB formula
was identified in pilot basin as the best perform-
ing concerning flood peak estimation.
Step 4: Design hydrograph: Flood peaks of
relevant frequencies for each sub-basin
Flood peak discharge is computed using a
simple rainfall-runoff model, as the rational
method (or kinematic method). Thus the flood
peak for a given RP will be computed as:
where Ф is the runoff coefficient, h the rainfall
height for given RP (reduced by ARF coefficient
as stated above), the basin area and tc the basin
concentration time.
For calibration analysis, maximum flood peaks
associated to given frequencies were estimated
from available observed discharge series in some
gauging station (or official estimates made avail-
able from MONRE or previous studies). Hy-
draulic parameters (CN, runoff coefficients
associated to different land use types) were cali-
brated to have a better representativeness in flood
peak estimation from DDF curves.
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